SCIENCE:

Severe flooding in India and wildfires in Colorado provided a backdrop of recent examples of weather-related disaster yesterday, as scientists and policymakers discussed the science of severe weather events at the American Geophysical Union's annual science policy conference in Washington, D.C.

The panel discussion led off with a straightforward yet disturbing fact: In 2012, the United States had the world's two costliest disasters, totaling more than $100 billion. One was Superstorm Sandy, and the other was the yearlong Midwest drought.

The linkage between climate change and these and other significant, costly weather events is one of considerable scientific and public policy interest.

A child's toy sits amid the wreckage of a home flattened by a tornado in Granbury, Texas, on May 15. Photo courtesy of Flickr.

While in some areas the science on this link is evolving, in other realms, such as the effects of sea-level rise on storm surges and flooding, the connection is clear, said Radley Horton, a scientist at Columbia University's Center for Climate Systems Research.

"The home run is the link between sea-level rise and climate change," Horton said.

On that front, New York City grows ever more vulnerable to flooding as the seas creep ever closer. If sea-level rise is on the high side of estimates, by 2050 coastal flooding risk in the area would increase fivefold, even without more severe storms, Horton said.

Another risk can come in the realm of heat waves. It's likely that the city will see a tripling in its number of 90-degree Fahrenheit days due to climate change after 2050, Horton added. "Just that shift in the averages has a profound influence on your likelihood of extreme events."

Some weather connections are less certain

Less certain is the connection between recent rapid melting of Arctic sea ice and changes in the jet stream that lead to severe weather events. A wavy jet stream has led to recent extreme events, like flooding in Europe and a record heat wave in Alaska, where temperatures were recently in the 90s.

But whether this phenomenon is being caused by Arctic sea ice melting, as some researchers have suggested, is still unclear. "Other things can give you a wavy jet stream as well," Horton said.

One way scientists can improve their understanding of this link is to improve their climate models. But by that time, because the Arctic is changing so fast, there might be an entirely different feedback mechanism in place, Horton noted.

Even without climate change, the impacts and the costs of severe weather events are increasing, explained Andrew Castaldi, a senior vice president at the reinsurance company Swiss Re.

That is because people continue to develop and build bigger homes and more valuable properties in risky areas, from the Florida coastlines to wildfire-prone parts of the American West. The link between such wildfires and climate change is still under discussion.

According to Janice Coen, who studies fire behavior at the National Center for Atmospheric Research in Boulder, Colo., fires are currently treated very simply in current climate models. There are lots of variables that can affect the severity of a fire, she said.

Drought and fires in the West; Northeast is too wet

Drought, which is often pointed to as a key driver of wildfire severity, is actually less important than things like the amount of dead fuel lying on the forest floor and local wind conditions, she said.

But climate change, by changing underlying conditions or lengthening the fire season, can also play a role. "Notorious wildfire events often occur at the intersection of both a climate anomaly and an unusual weather event," Coen added.

She pointed out that three sets of severe June wildfires in Colorado -- the 2002 Hayman fire, the 2012 High Park fire and the 2013 Black Forest fire -- all happened in drought years with unusually high winds in early June: "When the windstorms overlap a drought, then we get big fires."

In contrast to the dry western United States, the Northeast is facing threats from too much water. Sue Minter, the deputy secretary of Vermont's Agency of Transportation, displayed data showing that the Northeast has seen nearly a 70 percent increase in heavy rain events since 1958.

Minter outlined how her state had responded to the severe flooding of Hurricane Irene, saying the state is working to adapt its infrastructure so it is ready for the increase in severe weather that climate change may bring.

Minter concluded her talk with a call for leadership at the national level to address climate change, alluding to President Obama's speech on the topic. "I believe right now our president is discussing climate change. I hope that he will be courageous," she said.